Regions of the HIV envelope protein involved in T4 binding will be defined in direct and competition binding studies employing fragments of gp120 and T4 positive cells. We will also examine the mechanism of gp120/gp41 mediated cell fusion (1) by studying fusion of intact cells transfected with vectors expressing native and mutated forms of gp120/gp41 and (2) utilizing lipid vesicles reconstituted with HIV envelope proteins and envelope from other fusogenic viruses.